Prosecutors have added another murder charge against a California serial killing suspect, boosting the number of victims to 13 in the Golden State Killer case.

Former police officer Joseph DeAngelo, 72, was charged with first-degree murder in the killing of Claude Snelling, who was shot while stopping the kidnapping of his 16-year-old in Visalia, Tulare County district attorney Tim Ward said.

The community college teacher was killed in 1975.

"We have taken that first step in providing justice not only for the victim's family but for this community as a whole," Mr Ward said.

DeAngelo had already been charged in 12 killings throughout the state in the 1970s and 1980s that authorities say were committed by one of the state's most elusive serial killers.

Detectives are also confident that DeAngelo is a burglar known as the Visalia Ransacker, who struck more than 100 homes in the 1970s, terrorising the farming community about 40 miles south of Fresno, Visalia Police Chief Jason Salazar said.

DeAngelo worked as a police officer in the nearby town of Exeter from 1973 to 1976.

Investigators linked him to some of the killings by plugging DNA collected from a semen sample at one of the crime scenes into a genealogical website they say showed a match to a distant relative of DeAngelo.

Authorities say they then collected DNA from a tissue left in rubbish outside DeAngelo's house to make the final match.

Mr Salazar said Visalia investigators linked DeAngelo to the killing in that city through a description from a witness and physical evidence.

Authorities in northern California arrested DeAngelo in April at his Citrus Heights home and said they believed he was the killer who had long eluded authorities.

DeAngelo is also suspected of committing roughly 50 rapes but he cannot be tried for the rapes or burglaries because the statute of limitations has expired.